Gallium-arsenide (GaAs) based field-effect transistors can utilize a depletion region formed by a metal-semiconductor junction, commonly known as a Schottky junction, to modulate the conductivity of an underlying channel layer. Such devices have gained acceptance as a high performance transistor technology due to inherent physical properties of the gallium arsenide and related ternaries such as indium gallium arsenide (InGaAs). The devices are referred to by various names such as metal semiconductor field effect transistors (MESFET), high electron mobility transistors (HEMT), pseudomorphic high electron mobility transistor (pHEMT), two dimensional electron gas field effect transistors (TEGFET), and modulation doped field effect transistors (MODFET). Further details of the dynamics of charge transport in these structures can be found in Quantum Semiconductor Structures by Weisbuch, et al., 1991 by Academic Press, pages 38-55 and pages 141-154, which is incorporated herein by reference.
The basic gallium arsenide metal semiconductor field effect transistor, known as a MESFET, has the source and drain current carried via a relatively thin, highly doped, semiconductor layer, the channel. The current is controlled by the gate which forms a Schottky barrier on the semiconductor, and therefore, depending upon the applied gate voltage, depletes the semiconductor layer of electrons under the gate. Other devices such as the HEMT, pHEMT, and MODFET are based on the basic principles described above. The structure of a basic HEMT is based on the heterojunction between two dissimilar materials, AlGaAs (Aluminum Gallium Arsenide) and GaAs (Gallium Arsenide), which are well known to those of ordinary skill in the art. Ordinarily, the two dissimilar materials used for the heterojunction have the same lattice constant (i.e., spacing between the atoms).
The pseudomorphic HEMT or pHEMT is a HEMT where the two dissimilar materials used for the heterojunction do not have the same lattice constant. The formation of a heterojunction with materials of different lattice constants can be achieved by using an extremely thin layer of one of the materials—so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible, giving the transistors better performance through improved carrier confinement.
Essentially, the transistor structure consists of a semi-insulating substrate on which is first grown a buffer layer of nominally unintentionally doped GaAs. An n-doped layer of gallium arsenide, or pseudomorphic indium gallium arsenide, forms the channel for the device. An n-minus layer of AlxGa1-xAs is disposed on top of the channel layer to form a proper Schottky barrier with the gate metallization. The last layer is typically a GaAs contact layer which is doped highly n-type (n-plus) to facilitate the formation of ohmic contacts to the underlying channel layer. The two ohmic contacts disposed on this layer are generally referred to as the source and the drain contacts. Access resistances associated with the source and the drain contacts and the underlying semiconductor material to the intrinsic device are typically referred to as Rs and Rd, the source and drain resistances, respectively.
Electrons in the thin n-type AlxGa1-xAs layer move into the undoped gallium arsenide layer, forming a depleted AlxGa1-xAs layer. The electrons move into the undoped gallium arsenide layer because the heterojunction created by the two dissimilar (i.e., different band-gap) materials forms a quantum well in the conduction band on the lower band-gap gallium arsenide side. The electrons are confined in the conduction band quantum well and can move laterally with relatively low resistance due primarily to a reduction in the rate of impurity scattering. This creates a very thin layer of highly mobile conducting electrons with very high concentration. The high concentration of highly mobile conducting electrons give the channel very low resistivity (also known as high electron mobility). The very thin layer of highly mobile conducting electrons is commonly called a two-dimensional electron gas (2DEG).